The invention relates to a stabilizer for low ammonia natural rubber latices and latex compounds. More particularly, the invention relates to a method for improving the chemical stability of low ammonia natural rubber latices and latex compounds by the addition of a particular anionic phosphate surfactant which has been neutralized or partially neutralized to a pH of from 4 to 12 as measured in a 20 percent aqueous solution.
Natural rubber latices are used in a number of important applications such as, for example, prevulcanized dipping compounds, foams and molding compounds. Such latices are conventionally produced as concentrates by well known centrifuging or creaming processes. The latices are preserved or protected against bacterial attack by the addition of ammonia. Commercially available natural rubber latices are generally classified into two principal types. These are the so-called high ammonia (HA) types which generally contain 0.6 percent by weight or more of ammonia based on total weight of latex and the low ammonia (LA) types which generally contain 0.3 percent by weight or less of ammonia based on total weight of latex. In addition, the low ammonia types generally contain auxiliary preservatives such as zinc oxide, dithiocarbamates and thiuram disulfides.
High ammonia natural rubber latices are widely used in prevulcanized dipping compounds. However, such latices suffer from significant disadvantages. Thus, the high ammonia natural rubber latices upon drying liberate ammonia fumes to the atmosphere thereby contributing to environmental air pollution problems. In view of increasingly stringent air pollution standards, those active in the latex field have sought to limit the emission of ammonia fumes to the atmosphere. The most common approach to this problem has been to deammoniate the latex with formaldehyde. However, this approach not only adds to compounding costs but formaldehyde itself also contributes to environmental pollution problems.
In view of the foregoing, those active in the natural rubber latex field have devoted increasing effort to the development of low ammonia natural rubber latices. However, the low ammonia natural rubber latices also exhibit certain significant disadvantages which materially limit their usefulness in certain applications. Thus, for example, an important disadvantage of using such latices in the preparation of prevulcanized dipping compounds is that the resultant compound often lacks the necessary chemical stability to withstand prevulcanization conditions. This lack of chemical stability often results in gelation or coagulation of the latex compound during prevulcanization.
The lower than desirable chemical stability of low ammonia natural rubber latices in such applications is apparently related to the fact that such latices have a higher NH.sub.4.sup.+ /NH.sub.3 ratio than the high ammonia type latices. This leads to the formation of the lower order [Zn(NH.sub.4).sub.n ].sup.++ complex ions wherein n is 1 or 2 instead of 3 or 4. It is believed that under certain conditions, these zinc ammonia complex ions react with fatty acid soaps absorbed on the rubber particles to form insoluble zinc soaps. The resultant zinc soaps then apparently cause the latex to coagulate or gel.
Several approaches for improving the chemical stability of low ammonia natural rubber latices have been described or suggested in the prior art.
Early approaches to the problem involved a procedure intended to prevent the formation of insoluble zinc soaps by preventing the formation of the lower order [Zn(NH.sub.4).sub.n ].sup.++ complex ion wherein n is 1 or 2. In general, this was accomplished by adding KOH to the latex in sufficient amount to raise the pH of the latex to about 10 or more. The higher pH level of the latex tends to favor the formation of the less reactive higher order complex where n is 3 or 4. However, this approach has not achieved wide acceptance by users of natural rubber latices due to either inconvenience or actual or perceived difficulty in making such adjustments.
More recently, the prior art has suggested that the chemical stability of low ammonia natural rubber latices can be improved by utilizing certain anionic or non-ionic surfactants as stabilizers. However, it has been found that many anionic or non-ionic surfactants either do not provide sufficiently improved chemical stability, particularly in applications where severe processing conditions are encountered such as in the preparation of prevulcanized dip compounds or they adversely affect other properties of the compound such as dipping sensitivity, gel strength and the like.
Accordingly, those active in the natural rubber latex field are constantly seeking a method for improving the chemical stability of low ammonia natural rubber latices which does not involve complex compounding adjustments or the use of a stabilizer which with adversely affects other important properties of such latices or latex compounds.